diff --git a/.env.example b/.env.example
index e56b6a601..cdf6c9c01 100644
--- a/.env.example
+++ b/.env.example
@@ -84,9 +84,13 @@ RAG_AWAIT_QUERY_LOGS=false
 #RAG_PERSIST_RAW_QUERY_TEXT=false
 # Server-side key for the redacted query-hash placeholder (min 16 chars).
 # When set, stored query hashes are HMAC-SHA256 keyed pseudonyms — not
-# offline-reversible and not correlatable outside this deployment. Strongly
-# recommended wherever real clinical queries are logged. Changing or setting
-# the key changes future hashes, so historical dedup/joins reset from then on.
+# offline-reversible and not correlatable outside this deployment. REQUIRED in
+# production: instrumentation.register() calls requireQueryHashSecret() and the
+# server refuses to start without it, so clinical-query hashes are never logged
+# under the weak, reversible SHA-256 fallback (see docs/privacy-impact-assessment.md,
+# PIA-2). Generate one with:  node -e "console.log(require('crypto').randomBytes(32).toString('hex'))"
+# Changing or setting the key changes future hashes, so historical dedup/joins
+# reset from then on. Do NOT commit the real value.
 #RAG_QUERY_HASH_SECRET=
 
 # Private buckets created by supabase/schema.sql.
diff --git a/src/instrumentation.ts b/src/instrumentation.ts
index 58c82ad0c..1b22a6ccd 100644
--- a/src/instrumentation.ts
+++ b/src/instrumentation.ts
@@ -16,7 +16,7 @@ export async function register() {
     throw new Error("Refusing to start: local no-auth mode is enabled in a production build.");
   }
 
-  const { isDemoMode, requireOpenAIEnv, requireServerEnv } = await import("@/lib/env");
+  const { isDemoMode, requireOpenAIEnv, requireQueryHashSecret, requireServerEnv } = await import("@/lib/env");
 
   // A clinical production server must run against real, configured backends — never
   // in demo mode, which bypasses auth and serves canned content.
@@ -30,4 +30,8 @@ export async function register() {
   // is missing or points at the wrong project, instead of failing per-request.
   requireServerEnv();
   requireOpenAIEnv();
+
+  // A keyed HMAC secret must be present so clinical-query hashes written to the log
+  // tables are not reversible (PIA-2). Fail closed rather than degrade to weak SHA-256.
+  requireQueryHashSecret();
 }
diff --git a/src/lib/env.ts b/src/lib/env.ts
index 19fcfcab4..d802d6657 100644
--- a/src/lib/env.ts
+++ b/src/lib/env.ts
@@ -185,6 +185,20 @@ export function requireOpenAIEnv() {
   }
 }
 
+// Clinical query text is redacted to a keyed HMAC pseudonym before it is logged
+// (see query-privacy.ts). Without RAG_QUERY_HASH_SECRET the hash silently degrades
+// to an unsalted, dictionary-reversible SHA-256, which defeats the redaction: a
+// reader of the log tables can hash candidate patient/drug strings offline and match
+// rows. Production must fail closed rather than log real clinical queries under the
+// weak digest. See docs/privacy-impact-assessment.md (PIA-2).
+export function requireQueryHashSecret() {
+  if (!env.RAG_QUERY_HASH_SECRET) {
+    throw new Error(
+      "Missing RAG_QUERY_HASH_SECRET. It is required in production so logged clinical-query hashes are keyed HMAC-SHA256 pseudonyms, not offline-reversible SHA-256. Set a random secret (min 16 chars). See docs/privacy-impact-assessment.md (PIA-2).",
+    );
+  }
+}
+
 export function isDemoMode() {
   // Explicit opt-in is honored in every environment (e.g. a deliberate demo deploy).
   if (env.NEXT_PUBLIC_DEMO_MODE === "true") {
diff --git a/supabase/migrations/20260708120000_rag_query_misses_retention.sql b/supabase/migrations/20260708120000_rag_query_misses_retention.sql
new file mode 100644
index 000000000..723020bcf
--- /dev/null
+++ b/supabase/migrations/20260708120000_rag_query_misses_retention.sql
@@ -0,0 +1,55 @@
+-- PIA-4 (docs/privacy-impact-assessment.md): rag_query_misses stores the same
+-- hash-redacted query + candidate aliases as rag_queries, but unlike rag_queries
+-- (30d) and rag_retrieval_logs (90d) it had NO retention bound, so rows
+-- accumulated indefinitely. Add a matching nightly purge (90 days — wider than
+-- rag_queries to leave headroom for the miss-review workflow, still bounding
+-- long-lived query telemetry). Query text is already hash-redacted at write time
+-- via queryTextForStorage, so this bounds redacted telemetry, not raw PHI.
+set search_path = public, pg_catalog, pg_temp;
+
+create or replace function public.purge_expired_rag_query_misses(p_retention_days integer default 90)
+returns integer
+language plpgsql
+security definer
+set search_path = public, pg_catalog, pg_temp
+as $$
+declare
+  v_deleted integer;
+begin
+  if p_retention_days < 1 then
+    raise exception 'retention days must be positive';
+  end if;
+  delete from public.rag_query_misses where created_at < now() - make_interval(days => p_retention_days);
+  get diagnostics v_deleted = row_count;
+  return v_deleted;
+end;
+$$;
+
+revoke all on function public.purge_expired_rag_query_misses(integer) from public, anon, authenticated;
+grant execute on function public.purge_expired_rag_query_misses(integer) to service_role;
+
+comment on table public.rag_query_misses is
+  'Unresolved/low-confidence query telemetry (hash-redacted query + candidate aliases).
+   Rows older than 90 days are purged nightly by the pg_cron job
+   "purge-rag-query-misses" (see purge_expired_rag_query_misses). Adjust the window
+   by changing the interval in that job definition.';
+
+-- Register the nightly purge when pg_cron is available. Preview/branch databases
+-- may not ship pg_cron; install the function there but skip scheduling.
+do $cron$
+begin
+  if to_regnamespace('cron') is null then
+    return;
+  end if;
+
+  perform cron.unschedule(j.jobid)
+  from cron.job j
+  where j.jobname = 'purge-rag-query-misses';
+
+  perform cron.schedule(
+    'purge-rag-query-misses',
+    '45 3 * * *',
+    $job$select public.purge_expired_rag_query_misses(90);$job$
+  );
+end
+$cron$;
diff --git a/tests/env-query-hash-secret.test.ts b/tests/env-query-hash-secret.test.ts
new file mode 100644
index 000000000..7acec12be
--- /dev/null
+++ b/tests/env-query-hash-secret.test.ts
@@ -0,0 +1,29 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+// requireQueryHashSecret() reads the frozen `env` value parsed at import time, so
+// each case re-imports the module with a stubbed environment. Production must fail
+// closed when RAG_QUERY_HASH_SECRET is absent, so clinical-query hashes written to
+// the log tables are keyed HMAC pseudonyms and not offline-reversible SHA-256 (PIA-2).
+
+async function loadEnv(secret: string | undefined) {
+  vi.resetModules();
+  vi.stubEnv("RAG_QUERY_HASH_SECRET", secret);
+  return import("../src/lib/env");
+}
+
+afterEach(() => {
+  vi.unstubAllEnvs();
+  vi.resetModules();
+});
+
+describe("requireQueryHashSecret", () => {
+  it("throws when RAG_QUERY_HASH_SECRET is absent", async () => {
+    const { requireQueryHashSecret } = await loadEnv(undefined);
+    expect(() => requireQueryHashSecret()).toThrow(/RAG_QUERY_HASH_SECRET/);
+  });
+
+  it("does not throw when RAG_QUERY_HASH_SECRET is set", async () => {
+    const { requireQueryHashSecret } = await loadEnv("test-secret-at-least-16-chars");
+    expect(() => requireQueryHashSecret()).not.toThrow();
+  });
+});
diff --git a/tests/instrumentation.test.ts b/tests/instrumentation.test.ts
index fb47b1d3b..69b22ebee 100644
--- a/tests/instrumentation.test.ts
+++ b/tests/instrumentation.test.ts
@@ -17,6 +17,7 @@ const ENV_KEYS = [
   "SUPABASE_PROJECT_REF",
   "SUPABASE_PROJECT_NAME",
   "OPENAI_API_KEY",
+  "RAG_QUERY_HASH_SECRET",
   "NEXT_PUBLIC_LOCAL_NO_AUTH",
   "LOCAL_NO_AUTH",
 ] as const;
@@ -62,12 +63,23 @@ describe("instrumentation boot guard", () => {
     await expect(register()).rejects.toThrow(/OPENAI_API_KEY/);
   });
 
+  it("refuses to start a production server without a query-hash secret", async () => {
+    const register = await loadRegister({
+      ...PRODUCTION_NODE,
+      NEXT_PUBLIC_SUPABASE_URL: MATCHING_URL,
+      SUPABASE_SERVICE_ROLE_KEY: "service-role-key",
+      OPENAI_API_KEY: "openai-key",
+    });
+    await expect(register()).rejects.toThrow(/RAG_QUERY_HASH_SECRET/);
+  });
+
   it("starts a fully configured production server", async () => {
     const register = await loadRegister({
       ...PRODUCTION_NODE,
       NEXT_PUBLIC_SUPABASE_URL: MATCHING_URL,
       SUPABASE_SERVICE_ROLE_KEY: "service-role-key",
       OPENAI_API_KEY: "openai-key",
+      RAG_QUERY_HASH_SECRET: "test-secret-at-least-16-chars",
     });
     await expect(register()).resolves.toBeUndefined();
   });